Ignition composition

ABSTRACT

An igniter composition for a gas generator comprises a stoichiometric  comation of pyrotechnic metal selected from the group consisting of beryllium, boron, lithium, sodium, titanium, magnesium, aluminum and alloys and mixtures thereof and one or more halogenated polymers; and fumed colloidal silica in an amount from about 0.5 to about 2.5 percent of the total composition weight.

BACKGROUND OF THE INVENTION

The invention pertains generally to energetic materials and inparticular to pyrotechnic ignition materials.

Propellants, because of thermal-stability and insensitivityrequirements, are difficult to ignite. Ignition of these propellantsrequires the inclusion of a cartridge or pellet of a pyrotechnicmaterial.

The presently used pyrotechnic ignition materials comprise either boronand potassium nitrate or magnesium, polytetrafluoroethylene andpolytrifluorochloroethylene. The compositions have heat outputs of about1400 calories per gram, which, unfortunately, cannot reliably ignitesome propellants. To increase the energy of the ignition compositions,additives, similar to additives in high-energy propellants andexplosives, are added. Examples of ignition compositions with increasedheat are given in U.S. Pat. No. 3,753,811 by Julian et al.

A stoichiometric mixture of polytetraethylene and magnesium producesabout 2000 calories/gram which is sufficient to reliably ignite allknown propellants. This composition would be simple to process and wouldbe relatively inexpensive. Unfortunately, the composition is verydifficult to ignite and does not burn in stoichiometry.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a pyrotechniccomposition that is suitable for igniting any known propellant.

Another object of this invention is to increase the ignitability of thestoichiometric combinations of halogenated polymers and high-energymetals and maximize their combustion.

A further object of this invention is to increase the ignitability of astoichiometric combination of magnesium and polytetrafluoroethylene andmaximize its combustion.

These and other objects are achieved by the inclusion of not more than2.5 weight percent of fumed colloidal silica in a composition consistingessentially of one or more halogenated polymers and one or morehigh-energy metals.

DETAILED DESCRIPTION OF THE INVENTION

A possible mechanism for the surprising effect produced by the inclusionof a small amount of fumed colloidal silica or the combustion of one ormore halogenated polymers with one or more high-energy metals is thatthe silica produces a nearly homogenous distribution of the twocomponents. This mechanism is given by way of explanation and is notintended to limit this disclosure and the claims to follow in anymanner.

Since the highest energy is from the stoichiometric combination of ahigh-energy metal and polytetrafluoroethylene and this combination isextremely difficult to ignite and burn completely, the greatest benefitof the invention is realized with this combination. Accordingly, thepreferred halogenated polymer is polytetrafluoroethylene (PTFE). Othersuitable polymers include polytrifluorochloroethylene,polyhexafluoropropylene, and the copolymer of vinyledene fluoride andhexafluoropropylene. The preferred high-energy metal is magnesium. Othersuitable metals, including metalloids, have a heat of combustion foroxide formation of at least 4.0 kilo-calories per gram and arelightweight. These metals are beryllium, boron, aluminum, titanium, andalloys of these metals in which the above metals comprise at leastninety percent and have the requisite heat of combustion for the oxideformation. One such alloy is an alloy of two of the above metals, i.e.,magnesium and aluminum. The composition of the Mg-Al alloy that isparticularly important consists of 1 to 15 weight of aluminum and thebalance of magnesium.

The greatest energy is produced from a stoichiometric combination of ahalogenated polymer and a high-energy metal. Hence, preferredcompositions comprise stoichiometric or nearly stoichiometric amounts ofthese ingredients. Inclusion of silica would improve the combustion ofnonstoichiometric combinations also. Significant benefits are realizedwith polymer-metal combinations within about 10 to 12 percent ofstoichiometry. For a Mg-PTFE combination, the stoichiometric amount formagnesium is 33 weight percent and for polytetrafluoroethylene is 67weight percent. Hence, a composition of magnesium,polytetrafluoroethylene, and two weight percent of silica would consistof 32.3 weight percent of magnesium, 65.7 of polytetrafluoroethylene,and 2 weight percent silica. In general, a PTFE-Mg composition comprisesfrom about 31.5 to 34 weight percent of magnesium, from about 63.5 to 68weight of polytetrafluoroethylene, and from about 0.5 to 2.5 weightpercent of fumed colloidal silica. The preferred composition comprisesfrom 32.4 to 32.6 weight percent of magnesium, from 65.5 to 65.9 ofPTFE, and from 1.5 to 2 weight percent of silica.

Fumed colloidal silica is available under the tradename of Cab-O-Sil.The silica particles have an average particle size of less than aboutfive micrometers and are characterized by minute pores. It is preparedby a coagulation of hydrated silica. The amount of silica is from about0.5 to about 2.5 percent of total composition weight. The preferredamount is from 1 to 2 weight percent and the most preferred amount is1.5 to 2.0 weight percent. It is important that the silica has less thanabout 0.1 weight percent of water. The weight percent is based on totalsilica weight.

The preferred method of preparation comprises placing the powdered metaland halogenated polymer, both with an average particle size from 5 to 50micrometers with 10 to 50 micrometers preferred, in a blender; mixingfor at least about 30 minutes; removing relatively large agglomeratesby, e.g., screening; and mixing again for at least about 15 minutes. Thecomposition is then processed into the actual article of use, e.g.,tablet or cartridge.

To better illustrate the practice of the present invention and itsadvantages, the following examples are given. It is understood thatthese examples are given by way of illustration and are not meant tolimit the disclosure or the claims to follow in any manner.

EXAMPLE I

Dry magnesium powder (325 mesh), powdered DuPont Teflon No. 6, andCab-O-Sil in the respective amounts of 33 grams, 67 grams, and 2 gramswere tumbled in a P-K Twin-Shell blender for 45 minutes, screenedthrough 12 mesh, and tumbled for an additional 30 minutes. The powderappeared to have extremely uniform mix with no visible agglomerates.Five-gram pellets were pressed from the powder and tested.

The first test consisted of igniting two pellets by means of a match.Both pellets readily ignited. The second test was a ParrCalorimeter-Bomb test of three pellets to determine the amount of energyproduced. The results were 1982 cal/gm, 1987 cal/gm and 1979 cal/gm.

These results compared very favorably with the theoretical amount of2000 cal/gm and the consistency of the results demonstrates that thepowder mixture had a high degree of uniformity.

EXAMPLE II

EXAMPLE I was repeated except that no Cab-O-Sil was added. When themixture was screened, many more agglomerates were screened out than werescreened in EXAMPLE I. The mixture after the second mixing did notappear to be as uniform in appearance as the mixture in EXAMPLE I.

Ignition of three pellets by a match was not possible. One gram of acomposition comprising carboxy-terminated polybutadiene and about 88weight percent of ammonium perchlorate was combined with the pellets.The pellets ignited, but a sizeable residue remained after ignition,indicating that complete combustion had not occurred. The results fromthe calorimeter bomb tests were 1123 cal/gm, 1248 cal/gm and 1306cal/gm. The reduction in energy output indicated an inadequatecombustion and the inconsistency of the results indicated anonuniformity in the mix.

EXAMPLE III

EXAMPLE I was repeated except that 1.5 grams of Cab-O-Sil were added.The calorimeter results from two five-gram pellets were 1975 cal/gm and1988 cal/gm. The results were comparable to those in EXAMPLE I.

The results in the above examples demonstrate that the inclusion of asmall amount of fumed colloidal silica into a mixture of one or morehalogenated polymers and one or more high-energy metals significantlyincreases the ignitability and degree of combustion of the mixture. Theresults appear to confirm the hypothesis that nonuniformity is the causeof poor ignition of stoichiometric mixtures of halogenated polymers andhigh-energy metals.

Obviously, many modifications and variations of the Present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. An igniter composition for a gas generator comprises, on the basis of total composition weight, from about 97.5 to about 99.5 percent of a stoichiometric combination of a pyrotechnic metal selected from the group consisting of beryllium, lithium, sodium, titanium, magnesium, aluminum, boron and alloys and mixtures thereof and one or more halogenated polymers; and from about 0.5 to 2.5 percent of fumed colloidal silica.
 2. The igniter composition of claim 1 wherein said pyrotechnic metal is selected from the group consisting of magnesium, alloys of magnesium and aluminum wherein the amount of aluminum is from about 1 to 15 percent of the total alloy weight, and mixtures thereof.
 3. The igniter composition of claim 1 wherein said halogenated polymer is selected from the group consisting of fluoropolymers and fluorochloropolymers.
 4. The igniter composition of claim 2 wherein said halogenated polymer is selected from the group consisting of fluoropolymers and fluorochloropolymers.
 5. The igniter composition of claim 4 wherein said halogenated polymer is a fluoropolymer.
 6. The igniter composition of claim 4 wherein the amount of said silica is from 1 to 2 percent.
 7. The igniter composition of claim 5 wherein the amount of said silica is from 1 to 2 percent.
 8. The igniter composition of claim 5 wherein said halogenated polymer is a perfluoropolymer and said pyrotechnic metal is magnesium.
 9. The igniter composition of claim 8 wherein the amount of said silica is from 1.5 to 2.0 percent.
 10. The igniter composition of claim 9 wherein said halogenated polymer is polytetrafluoroethylene.
 11. An igniter composition for a gas generator comprises, on the basis of total composition weight, from about 31.5 to 34 percent of magnesium; from about 63.5 to 68 percent of polytetrafluoroethylene, and from about 0.5 to 2.5 percent of fumed colloidal silica.
 12. The igniter composition of claim 11 wherein the amount of polytetrafluoroethylene is from 32.4 to 32.5 percent and the amount of fumed colloidal silica is from 1.5 to 2.0 percent. 